These arrange pressure vessels filled with membrane elements in
such a way as to maximize the quality of pure water produced. The purified “permeate” water from the first pass becomes the feed to the second pass. The first stage/pass usually has twice as many pressure vessels (and membrane elements) as the second stage/pass. These systems are commonly found in the microelectronics industry where the purity of rinse water is essential to production. Three-pass systems are the most common.